The present invention relates to a screening apparatus for grains, seeds or the like crops comprising a housing wherein a support structure is arranged, which is adapted to be rotated about a main axis and to support screening means adapted to be rotated about a planetary axis eccentrically arranged with respect to the main axis, the support being provided with vane means which during the rotation of the support structure move in a delivery channel for the screened goods, formed in the housing.
An apparatus as referred to above is known from Danish patent specification No. 122,791. The screening means of this known apparatus consist of two screen drums, arranged diametrically with respect to the main axis and, accordingly, the planetary axes of the drum screens have an eccentricity with respect to the main axis which is greater than the longest radius of the screen drums, because the screen drums otherwise would interfere with each other. Moreover, such arrangement of screen drums is characteristic for the previously known screening apparatuses of the kind referred to above.
During the operation of such screening apparatus, the support structure and, accordingly, also the screen drums are driven in a planetary movement about the main axis and, simultaneously, each of the screen drums performs a rotating movement about its own axis which, accordingly, constitutes a planetary axis. Accordingly, the material which is fed into the screen drums will be subjected to a centrifugal force which results in that a body of material, having a cross section shape generally corresponding to a kidney, will adjust itself along the parts of the screen drums which, at any time, face outward and such body of material will move comparatively slowly down through the corresponding drum, and, simultaneously, the material which constitutes the body will be tumbled around upon the inner surfaces of the screen means whereby the screening effect wanted is achieved.
In order that the vane means which move in the delivery channel for the screened goods may conduct their dispensing effect satisfactorily, the vane means have to keep a certain peripheral speed and, simultaneously, have to be positioned at a certain distance from the main axis. Moreover, in order to achieve a reasonable good screening effect, the screening means must keep a peripheral speed which lies within comparatively narrow limits. If the peripheral speed of the screening means is too high, the material to be screened through the apertures of the screening means has a liability to "jump" over the apertures and, in case the peripheral speed of the screen means becomes very high, the bodies of material may have a tendency to move together with the screen means instead of being tumbled around on the screen means. However, on the other hand, if the peripheral speed of the screen means is too low, the screening effect will be drastically reduced and the material gets a liability to pass down through the drums at a speed which is too high to allow the material to be screened, really to be screened through the screen means.
Provided the operation conditions explained above are observed, the bodies of material will achieve a kidney shaped cross section as explained above which, however, is comparatively short as seen in the circumference direction of the screen means but which, on the other hand, has a comparatively large thickness in the radial direction. Said in other words, the kidney shape becomes short and chubby. However, this means that only a comparatively small part of the screen means will be active at any time because it will be understood that the screening effect will only be effected along the outer surface of the kidney shape which, as explained above, is comparatively narrow.
It is the object of the present invention to provide a screening apparatus as defined above wherein it is possible to keep the optimum speeds both as regards the vane means and as regards the screen means and wherein a comparatively broad kidney shape, having a comparatively small thickness in the radial direction, simultaneously, is formed whereby the active part of the screening means will be increased.